Some car navigation systems can designate a detour to a destination. (Refer to Patent 1: JP-A-H8-233589 (U.S. Pat. No. 6,118,389).) According to a car navigation disclosed in Patent 1, a route from a star point to an end point as a destination is set by using evaluation values assigned to links constituting a road map. A detour for the route is calculated by lowering the evaluation values of the links included in the previously set route to prevent the same from being set again.
However, in the above method for setting a detour, all the evaluation values are simultaneously lowered in all the links within a given region ahead in the route to the end point. For instance, even when a user wants to use the same route at least in regions surrounding a start point and a destination, with using the above method a different route is undesirably designated in the regions surrounding the start point and the destination. To solve this problem, a method sets a route by using evaluation values that are changed based on a traveling length (or traveling distance) from the start point or the destination. (Refer to Patent 2: JP-A-H8-292058.)
According to a car navigation system disclosed in Patent 2, position relation is considered within a previously set route. Namely, costs of the links, which act as negative evaluation values, around a linear center (or around an intermediate region) of the route are remarkably changed with increasing multiplying powers of coefficients, while costs in the regions around the start point and the destination are hardly changed. Differentiation from the previous route is thereby strengthened around the linear center of the route while being prevented in the regions around the start point and the destination. This leads to designating a detour that is differentiated around the linear center of the previously set route.
However, in the above method, as long as a major road such as an expressway or a national road is not located around the linear center of a route, the major road cannot be eventually avoided.